During the last twenty years thermosetting powder paints have replaced solvent-based liquid thermosetting paints, resulting in considerable advantages in ecological terms owing to elimination of the solvent in which the solid components forming the coating paint were dissolved.
The methods used envisage so-called fluid beds, inside which the powder paint is kept suspended inside a container, by means of a homogeneously distributed carrier gas (air), and the pre-heated metal part to be painted passes through this powder haze.
The powder melts on the surface of the part and adheres to it.
In the case of oven treatment, the powder undergoes a thermosetting reaction and becomes solid and fused with the part.
Electrostatic spraying plants which imitate the abovementioned plants for performing liquid painting with solvent paints are widely used.
These plants consist of a part similar to the abovementioned fluid bed, inside which the powder paint is kept mixed with a gas (air).
This mixture is then transferred by means of pipes to a powder spray gun inside which an expelling thrust is imparted to this mixture by means of a Venturi tube.
During transfer, the rubbing of two particles together produces a large quantity of electrostatic charges which accumulate on the ends of the powder paint particles, thus forming an electrostatic charge.
A magnetic field is generated between the tip of the spray gun and the part to be painted, this field having the effect that the particles, during their propelled movement, move along magnetic field lines.
A more recent technology increases this movement of the particles under the influence of the magnetic field in that it creates a difference in potential of more than 100,000 Volts between the needle of the application nozzle and the part to be painted.
The movement in these magnetic fields gives rise to various phenomena:                particles which are similar, but have a different specific weight, follow different paths;        particles with different shapes, and hence with a varying charging capacity, follow different paths;        particles with different dimensions follow different paths;        particles with a different chemical composition are charged differently and follow different paths;        particles which are entirely identical in terms of composition and form, but which receive a different thrusting force, move along different paths.        
All of this, which has been widely established, has led paint manufacturers to produce formulations where all the powder paint components undergo a premixing step which is performed before extrusion of all the components, in order to render them uniform and homogeneous, and are then ground using mills of varying types so as to convert them into the powder state.
A powder which is homogeneous in terms of its composition, specific weight and form, but non-homogeneous in terms of size, is thus obtained.
With these characteristics, the powder paint is suitable for coating the article in a homogeneous manner, except for the surplus of a small part of powder paint which, owing to its particle-size distribution, cannot be easily transferred onto the part.
The larger particles which are propelled with a greater kinetic energy escape from the electrostatic field and travel further.
The particles which are too fine either do not receive sufficient energy to reach the part or along the path are attracted also by other elements and fall onto the bed of the spraying booth.
In any case that part which is deposited onto the part is homogeneous and, after melting, forms a homogeneous film of paint, where the various pigments, to the naked eye, cannot be distinguished as a heterogeneous system, exactly in the manner of liquid paints.
Consequently the blue particles and yellow particles of a green will not be visible, but instead a green homogeneous surface.
The result would not be so if, instead of applying this powder paint, a mixture of two powder paints were to be prepared in the manner described above and applied, i.e. a yellow coloured paint and a blue coloured paint mixed together and then applied.
The mixture of the two powder paints, with a different specific weight and different particle-size curve, under the effect of the magnetic field is transferred onto the part with a composition different from that supplied, producing a shade different from that intended.
Moreover, the continuous varied consumption in the end will result in one of the two paints being left over.
Furthermore, the large-size particles normally used, ranging from 20 to 100 microns and the binders normally used with a low molecular mobility, such as polyesters or epoxy polyesters or epoxides or acrylics which must impart functional properties to the powder paint, are such that the surface of the painted part will not have a homogeneous green colour, even if the transfer had been performed in a perfect manner, but will appear like a paint broken down into blue and yellow parts which are very small, but visible.